Certain electrical devices, e.g., tungsten halogen lamps, employ envelopes comprising a high percentage of silica, such as fused silica, fused quartz or Vycor, the latter being a 96% silica glass. To achieve a hermetic seal between the glass and the lead-in conductors of the electrical device it is common practice to employ a pinch seal. The lead-in conductors generally comprise a very thin foil portion of molybdenum which forms the actual hermetic seal and an outer lead-wire portion of a refractory metal which can also be molybdenum. One end of the lead-wire is attached to the foil and the other end extends exteriorly of the pinch. This relatively heavy outer lead-wire does not form a hermetic seal with the quartz because of its relatively large diameter and the difference in thermal expansion coefficients between the two. In actual practice, because the formation of the pinch seal does not allow the quartz to flow completely around and against the full periphery of the outer lead-wire, a small capillary passage is left therebetween. The thin foil is thus exposed to atmospheric oxygen entering via the capillary passages. At elevated temperatures, say above about 350.degree. C., oxidation of the foil can occur, resulting in breakage of the electrical connection to the outer lead-wire.
It has been suggested that this problem can be reduced by filling the capillary with a solder glass which becomes molten when the device is operated, thus forming a molten seal. Specifically, a lead borate glass has been suggested; however, the use of such a glass requires the use of platinum or platinum clad lead-wires since lead borate attacks molybdenum. Further, U.S. Pat. No. 3,588,315 suggests binary glasses such as antimony borate and ternary glass compositions of antimony borate with the addition of small amounts of molybdenum trioxide or tungsten trioxide.
It would be an advance in the art if additional solder glasses could be made available. It would be a further advance if such glasses were opaque to visible radiation.